1. Field of the Invention
The present invention relates to a method of forming a glass film on a semiconductive wafer or circuit utilizing sedimentation of a suspension of glass particles from a dielectric fluid. More particularly, the present invention relates to a method of forming a glass film on a semiconductive or electronic device, such as an integrated circuit or thin or thick film device, including the step of adding a preferentially adsorbed substance and centrifuging the treated dielectric fluid whereby a more cohesive and desirable glass film may be formed at lower centrifugal forces.
2. Description of the Prior Art
The deposition of thin glass films and in particular, electrically passivating coatings on various articles of electronic manufacture, is an important and long practiced art. The prior art forms thin, uniform glass coatings on semiconductive or electronic articles of manufacture by a variety of means, e.g. sputtering, evaporating, or condensation (silox process). In addition the prior art has devised several methods whereby sedimentation from a suspension of glass particles from a fluid is achieved by means of centrifugal force. For example, Pliskin in U.S. Pat. No. 3,212,921 (1965) discloses a method whereby glass particles are suspended in a high dielectric fluid, centrifuged onto the desired substrate, removed from the centrifuged slurry and then heated to fuse the glass layer into an integral coating. This method uses a slurry containing glass particles generally in the range of 0.1-2 microns in diameter; and centrifuge speeds capable of developing from 1000 to 2500 times the force of gravity (1000-2500 g). These high centrifugal forces are in fact so great as to cause the particles to settle from the slurry in substantially straight trajectories. This projectile manner of sedimentation results in a poor covering of mesa or step structures at the surface of the object. In other words, any nontrivial, three dimensional projection from the planar surface of the object casts a shadow or creates an undercut surface which is poorly or substantially not covered by the settling glass particles. In addition to these disadvantages the prior art centrifugal sedimentation method produces a glass film which is only weakly self-cohesive and thus tends to "run out" or to spill from the surface of the coated object upon the slightest disturbance or turbulence of the overlying fluid. The high centrifuging force further causes the glass film to "run out" if the surface of the coated object is inclined with respect to the direction of the centrifugal force.
The prior art has attempted to solve the problems of "run out" during centrifuging or decanting by a method wherein two layers of dielectric fluid are utilized. See Pliskin, in U.S. Pat. No. 3,212,929 (1965). According to this prior art method two miscible dielectric fluids are placed one over the other to form two layers over the object to be coated. The upper layer is a less dense fluid having a higher dielectric constant whereby, as explained below, the colloidal nature of the suspension is improved. The lower layer is a denser fluid having a lower dielectric constant whereby the mutually repelling forces of the particles are lessened and a glass layer is formed which is somewhat more resistant to "run out". Even with the two layer method great care must be taken when adding or decanting the fluids, and it is still necessary to centrifuge the slurry between 1000 to 2500 g, in order to induce sedimentation. In addition, the problem involving coverage of shadow areas caused by steps or mesas on the surface of the object is ignored. It was later found by Pliskin, U.S. Pat. No. 3,505,106 (1970) that the two layer sedimentation process can be replaced in some cases by a process in which a glass suspension, carried in a high dielectric fluid, is diluted by a second dielectric fluid having a low dielectric constant. As a result the dielectric constant of the mixture of the two is approximately intermediate. Thus the particles once suspended in the high dielectric media could be induced to settle more readily from the slurry and be somewhat more resistant to "run out" if the dielectric constant of the slurry could be lowered, which in this case is acheived by dilution by a low dielectric fluid.
A further attempt by the prior art to avoid the problems of "run out" involves the construction of suitable apparatus fitted into the centrifuging cup, whereby the object to be coated slowly rises and lowers into the dielectric fluid in proportion to the amount of applied centrifugal force. The rising and lowering of the object is accomplished by placing the object within the centrifugal cup on a spring loaded platform. Centrifugal force compresses the spring causing the platform to be lowered into the dielectric fluid. Likewise, the spring slowly raises the platform from the fluid as the centrifugal force lessens. See Conrad, in U.S. Pat. No. 3,406,041 (1968). While this apparatus may be a convenient mechanical means for removing the coated object from the fluid in lieu of decanting the fluid, it still does not address the problem of centrifuged "run out" or shadow coverage. Therefore, what is needed then is a method for forming a glass film on an object such that a film may be formed which is resistant to "run out" of the type caused by centrifuging or turbulence during decanting; and which method is capable of forming a flowable glass film thereby eliminating uncovered shadow or undercut areas.